The invention is directed towards an improved security document and, more particularly, to a security document containing machine readable code.
Color photocopiers have been used for years to make accurate copies of commonly available documents. In many cases, there are legitimate reasons for making such copies. Unfortunately, color copies may also be made and used for illegal purposes. Specifically, there has been concern that color copiers could be used to reproduce security documents, such as checks, stock certificates, automobile title instruments, birth certificates, college transcripts, prescriptions, and other documents of value, for illegal purposes. This concern has been heightened with the advent of desk top publishing software and hardware, including personal computers and scanners. Such desk top publishing systems allow sophisticated image processing and printing not previously generally available.
Many techniques have been developed to prevent improper reproduction of security documents. One of the most successful is the use of a hidden warning message which is readily apparent on reproduced copies of a document, but which is invisible, or nearly so, on the original document.
Many techniques have been used to produce this effect. One technique is shown in U.S. Pat. No. 4,227,720. A single tone warning phrase and a single tone background pattern are used. Tone refers to the visual effect produced by solid ink coverage or by halftone dots, bars, or marks which cover a portion of a printed area and which usually have a frequency that is measured in dots, lines, or marks per inch. Halftone dots, bars, or marks printed with a dark ink may be more or less uniformly distributed over an area to produce the visual effect of a lighter overall color. Printing an image with less than full area coverage is said to be printing less than a full tone image. The warning phrase and background pattern area tones are of different frequency and are made up of dots, bars, or marks of differing size, but they are selected to provide similar appearance to the eye of a casual observer. A less than full tone effect may also be produced by full area coverage of a paler color of ink than the darker color of ink used for the halftone dots, bars, or marks.
Because the tone of the warning phrase and the tone of the background pattern are selected to be generally the same, these two areas have much the same visual impact on an observer of the original document, and the warning phrase is not readily perceived. The optics of color copiers have typically been unable to reproduce relatively small halftone dots, lines or other elements. As a consequence, reproduced copies of the original document will have a noticeable warning phrase.
A camouflage pattern is sometimes utilized to obscure the warning phrase further. The camouflage pattern may be defined by areas in which the individual dots, bars, or marks have been completely or partially deleted from both the warning phrase and the background pattern. The camouflage pattern may also be defined by a pattern of dots, bars, or marks which are smaller than or larger than those used in the background pattern and the warning phrase, or by areas of complete coverage of a paler ink. The camouflage pattern may permit the tone of the warning phrase and the tone of the background pattern to differ somewhat, while confusing the eye of the casual observer so that the warning phrase is not readily apparent.
The U.S. Pat. No. 4,227,720 patent uses small dots as background elements while larger dots are used to form a warning word. Other patents have used different elements to achieve a similar effect. U.S. Pat. No. 4,891,666 uses small dots as background elements and line segments to form a warning word. U.S. Pat. No. 5,375,886, on the other hand, uses curved lines as background elements and small dots to form the warning word. Another technique is shown in UK Patent Application GB 2,018,197 A. In this published application, line segments are used both as background elements and as warning word elements. The lines are perpendicular to each other in the areas defining the background pattern and the warning phrase.
In recent years, color copiers have been improved substantially. These new color copiers have made the above techniques less effective in protecting documents. By manipulating the control settings on such copiers, copies can be made of such documents in which the warning phrase does not appear on reproductions when some of the most commonly used frequency and size combinations are used. For example, by adjusting the settings for sharpness and lightness/darkness it has still been possible on some copiers for a skilled individual to produce a copy in which the warning phrase is not visible. Furthermore, desk top publishing systems now available in conjunction with laser printers, offer additional possibilities for unauthorized copying.
Therefore, there remains a need in the art for a security document which provides improved protection against copying over a wide range of copier settings, or against manipulation using desk top publishing systems.